It is desirable to be able to determine the location (that is, the position coordinates relative to some space-fixed system) and classification or identity (that is, the characteristic radii, diameters, lengths, shape, dipole or higher order moment, etc.) of a ferrous object, given that the object type (magnetic dipole, sphere, spheroid, rod, pipe, rope, cable, etc.) is known and that magnetic and positional measurements must be made in a planar surface underneath or behind which the object is buried. By way of example, it is desirable to be able to determine the position in a plane, depth of burial, and inner and outer radii of a ferrous rod, rope, pipe or cable (hereafter referred to as a "rod") which is buried a constant depth beneath a planar surface, by making magnetic measurements on the planar surface, the term depth referring to the minimum distance from the rod axis to the plane. This capability is important in a number of applications:
a. The field identification of the various types of structural concrete members for demolition tasks. Since all tensile loads are carried by the imbedded steel, usually in the form of bars, rods, wire rope or cable, explosive demolition relies on weakening the steel or the sections of concrete where the steel is absent. Knowledge of the size, depth of burial, and orientation of the steel rods is therefore important for effecting successful demolition of the structure while using a minimum of explosive materials.
b. The location and identification of steel or iron conduits buried in floors and walls by the construction industry. Such information can save the costly dismantling of walls or floors by accurately locating a specific conduit.
c. The location and identification of steel or iron pipe bombs by police forces and the military. Such information can distinguish between a conduit and a bomb in a floor, wall or culvert, and give an estimate of the size of the bomb.
d. The location and determination of the radius of steel gas pipes by utility companies. Accurate location and identification can substantially reduce digging costs.
It is also desirable to be able to determine the location, including depth of burial and magnetic dipole moment components, of a static magnetic dipole which is buried beneath a planar surface, by making magnetic measurements on the planar surface. This problem arises in a number of applications where the object of interest is composed of ferrous material and measurement is made at distances of at least a few times a characteristic dimension of the object; for example:
a. Detecting small ferrous objects imbedded in a human body. Bullets, steel slivers or other ferrous fragments are often difficult to locate if ingested or otherwise imbedded in the body. Such objects normally are small enough that they can be considered magnetic dipoles if magnetic measurements are made a few centimeters above the area of interest.
b. Detecting the location and estimating the size of a ferrous object during archaeological investigations, without disturbing the object or the surrounding environment.
c. Detecting handguns in baggage or on persons, by estimating the amount of steel present.
It is also desirable to be able to determine the location and radius of a sphere or location and length of major and minor axes of a ferrous spheroid buried beneath a planar surface, by making magnetic measurements on the planar surface. This problem can arise in a number of applications, most notably in the detection of foreign ferrous bodies of such shape in the human body, and in the detection of small ferrous explosive objects under floors or behind walls.
A number of detectors are available which measure small magnetic fields with sufficient sensitivity to detect ferrous rod, magnetic dipoles, ferrous spheres, spheroids, etc. of sizes and depths of burial typical of the applications mentioned above. Such detectors are called magnetometers, and their output signals or values are a function of both relative sensor-to-object distance and object size, shape and orientation. These components of the magnetometer output cannot readily be separated, since it is necessary to obtain sensor position information, in addition to the magnetic information, in order to be able to separate the components. Currently available magnetometers do not have the ability to collect sensor position and magnetic data and analyse the data with a microprocessor or computer in realtime, and thus no detector is capable of directly yielding the position coordinates and size and shape parameters.